Cogging torque measuring method for motor

Abstract

A method for determining a cogging torque may include applying a substantially constant rotation torque to an output shaft of a motor without electrically energizing the motor; detecting, an angle of rotation of the output shaft of the motor at each of a plurality of angles of rotation; calculating an angular acceleration of the motor at each of the plurality of angles of rotation by second order differentiating the angle of rotation; calculating a measured torque value at each of the plurality of angles of rotation by multiplying the angular acceleration by a moment of inertia of a rotor in the motor; obtaining a measured torque waveform of the measured torque value at each of the plurality of angles of rotation as a function of the plurality of angles of rotation; and calculating a cogging torque waveform based on a frequency of the measured torque waveform.

What is claimed is: 
     
       1. A method for determining a cogging torque of a motor, comprising:
 manually applying a substantially constant rotation torque to an output shaft of the motor to rotate the output shaft of the motor without electrically energizing the motor; 
 detecting, using a rotation angle detector, an angle of rotation of the output shaft of the motor at each of a plurality of angles of rotation; 
 calculating an angular acceleration of the motor at each of the plurality of angles of rotation by second order differentiating the angle of rotation; 
 calculating a torque value at each of the plurality of angles of rotation by multiplying the angular acceleration at each of the plurality of angles of rotation by a moment of inertia of a rotor in the motor; 
 obtaining a torque waveform in which the torque value at each of the plurality of angles of rotation is plotted as a function of the plurality of angles of rotation; and 
 calculating a cogging torque component for each order of rotation based on a frequency of the torque waveform. 
 
     
     
       2. A method for determining a cogging torque of a motor, comprising:
 attaching a weight to a tip of a wire that is wound around an output shaft of the motor, the weight being selected to exceed a static frictional torque of the output shaft of the motor; 
 rotating the output shaft of the motor without electrically energizing the motor by dropping the weight to pull the wire; 
 detecting, using a rotation angle detector, an angle of rotation of the motor at each of a plurality of angles of rotation; 
 calculating an angular acceleration of the motor at each of the plurality of angles of rotation by second order differentiating the angle of rotation; 
 calculating a torque value at each of the plurality of angles of rotation by multiplying the angular acceleration by a moment of inertia of a rotor in the motor; 
 obtaining a torque waveform in which the torque value at each of the plurality of angles of rotation is plotted as a function of the plurality of angles of rotation; and 
 calculating a cogging torque component for each order of rotation based on a frequency of the torque waveform. 
 
     
     
       3. A method for determining a cogging torque of a motor, comprising:
 directly connecting a direct drive motor to an output shaft of the motor; 
 rotating the output shaft of the motor without electrically energizing the motor by driving the direct drive motor in accordance with a constant torque command to make an output torque from the direct drive motor constant; 
 detecting, using a rotation angle detector, an angle of rotation of the motor at each of a plurality of angles of rotation; 
 calculating an angular acceleration of the motor at each of the plurality of angles of rotation by second order differentiating the angle of rotation; 
 calculating a torque value at each of the plurality of angles of rotation by multiplying the angular acceleration at each of the plurality of angles of rotation by a moment of inertia of a rotor in the motor; 
 obtaining a torque waveform in which the torque value at each of the plurality of angles of rotation is plotted as a function of the plurality of angles of rotation; and 
 calculating a cogging torque component for each order of rotation based on a frequency of the torque waveform.